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<a name="Memory-Management-and-Type-Information"></a>
<h2 class="chapter">23 Memory Management and Type Information</h2>
<a name="index-GGC"></a>
<a name="index-GTY"></a>

<p>GCC uses some fairly sophisticated memory management techniques, which
involve determining information about GCC&rsquo;s data structures from GCC&rsquo;s
source code and using this information to perform garbage collection and
implement precompiled headers.
</p>
<p>A full C++ parser would be too complicated for this task, so a limited
subset of C++ is interpreted and special markers are used to determine
what parts of the source to look at.  All <code>struct</code>, <code>union</code>
and <code>template</code> structure declarations that define data structures
that are allocated under control of the garbage collector must be
marked.  All global variables that hold pointers to garbage-collected
memory must also be marked.  Finally, all global variables that need
to be saved and restored by a precompiled header must be marked.  (The
precompiled header mechanism can only save static variables if they&rsquo;re
scalar. Complex data structures must be allocated in garbage-collected
memory to be saved in a precompiled header.)
</p>
<p>The full format of a marker is
</p><div class="smallexample">
<pre class="smallexample">GTY (([<var>option</var>] [(<var>param</var>)], [<var>option</var>] [(<var>param</var>)] &hellip;))
</pre></div>
<p>but in most cases no options are needed.  The outer double parentheses
are still necessary, though: <code>GTY(())</code>.  Markers can appear:
</p>
<ul>
<li> In a structure definition, before the open brace;
</li><li> In a global variable declaration, after the keyword <code>static</code> or
<code>extern</code>; and
</li><li> In a structure field definition, before the name of the field.
</li></ul>

<p>Here are some examples of marking simple data structures and globals.
</p>
<div class="smallexample">
<pre class="smallexample">struct GTY(()) <var>tag</var>
{
  <var>fields</var>&hellip;
};

typedef struct GTY(()) <var>tag</var>
{
  <var>fields</var>&hellip;
} *<var>typename</var>;

static GTY(()) struct <var>tag</var> *<var>list</var>;   /* <span class="roman">points to GC memory</span> */
static GTY(()) int <var>counter</var>;        /* <span class="roman">save counter in a PCH</span> */
</pre></div>

<p>The parser understands simple typedefs such as
<code>typedef struct <var>tag</var> *<var>name</var>;</code> and
<code>typedef int <var>name</var>;</code>.
These don&rsquo;t need to be marked.
</p>
<p>Since <code>gengtype</code>&rsquo;s understanding of C++ is limited, there are
several constructs and declarations that are not supported inside
classes/structures marked for automatic GC code generation.  The
following C++ constructs produce a <code>gengtype</code> error on
structures/classes marked for automatic GC code generation:
</p>
<ul>
<li> Type definitions inside classes/structures are not supported.
</li><li> Enumerations inside classes/structures are not supported.
</li></ul>

<p>If you have a class or structure using any of the above constructs,
you need to mark that class as <code>GTY ((user))</code> and provide your
own marking routines (see section <a href="User-GC.html#User-GC">User GC</a> for details).
</p>
<p>It is always valid to include function definitions inside classes.
Those are always ignored by <code>gengtype</code>, as it only cares about
data members.
</p>
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<tr><td align="left" valign="top">&bull; <a href="GTY-Options.html#GTY-Options" accesskey="1">GTY Options</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">What goes inside a <code>GTY(())</code>.
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<tr><td align="left" valign="top">&bull; <a href="User-GC.html#User-GC" accesskey="3">User GC</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Adding user-provided GC marking routines.
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<tr><td align="left" valign="top">&bull; <a href="GGC-Roots.html#GGC-Roots" accesskey="4">GGC Roots</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Making global variables GGC roots.
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<tr><td align="left" valign="top">&bull; <a href="Files.html#Files" accesskey="5">Files</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How the generated files work.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Invoking-the-garbage-collector.html#Invoking-the-garbage-collector" accesskey="6">Invoking the garbage collector</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to invoke the garbage collector.
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<tr><td align="left" valign="top">&bull; <a href="Troubleshooting.html#Troubleshooting" accesskey="7">Troubleshooting</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">When something does not work as expected.
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